Some fabrics having multiple layers are formed as knit spacer fabrics, in which two knit fabric layers are knit in parallel so as to have threads extending between the layers, so as to give the overall fabric more loft. Such fabrics may be produced, for example, on a weft knitting machine having two sets of needles that produce two distinct layers of fabric that are held together by tucks, or on double bar raschel warp knitting machines. Other multi-layer fabrics are formed by laminating (such as by adhesive) two pre-formed fabric layers.
Multi-layer fabrics have many uses, including in the medical, apparel and sports markets for applications in which one of the fabric layers is to be placed against skin. In such applications it is desirable that the fabric layer to be placed against skin be comfortable to the touch. For applications in which the fabric is to engage with touch fasteners, the outer layer may be constructed to be hook engageable. Such products are useful in medical and sports markets for applications such as straps, which may support medical devices on a patient, or in braces that may be wrapped around body parts, such as elbows or knees as a support.
For some applications, hook-engageable loop fabrics are laminated to foams or Neoprene polychloroprene to add comfort, stretch, or compress-ability. Many of these composite fabrics have poor air permeability and do not allow sweat and heat to move quickly away from the skin. In prolonged skin contact situations, bacteria can develop in this wet environment, resulting in odor, possible skin infections or dermatitis. In some products, the fabric used on the inside of the laminate in contact with the skin is an inexpensive tricot knit fabric.
In some laminates, such as Breath-O-Prene®, a hook-engageable fabric is laminated to one side of a Neoprene polychloroprene, and an inexpensive tricot fabric is laminated on the other side. The Neoprene polychloroprene in this case is perforated in an attempt to promote air penetration and moisture movement. Silver-Tec® brand laminate is similar to a Neoprene polychloroprene laminate and contains silver to provide anti-microbial properties. Some other straps, such as the Futuro™ brand straps made by 3M, feature an open, net-like fabric such as in the Powerstretch-Rx® product available from Polartec. This fabric is a non-hook-engageable knit fabric that is said to have moisture-releasing properties. Other brace straps, like the Futuro® Infinity precision-fit braces, advertise enhanced breathability and anti-microbial support. These laminates are foam laminates with a soft interior skin contact fabric laminated to a foam core, then laminated to an outer layer of hook-engageable loop material with holes on one side of the laminate, such as formed by knitting the material with multiple tucks on one needle, that allow for some air and moisture transport.
Single-knit jersey fabrics are generally inexpensive and found in such things as underwear and T-shirts. Due to their interconnected loop structure, knit materials in general can be deformed or stretched by elongating individual stitches, even when the fabric is knit of yarns made of non-elastomeric fiber materials. To provide a greater degree of stretch and stretch recovery, elastomeric yarns may be knit into a fabric. One type of elastomeric yarn in common use is spandex. Spandex, sometimes sold under the trade name LYCRA®, is a manufactured fiber of a long-chain synthetic polymer containing at least 85 percent segmented polyurethane. The polyurethane is prepared from a polyether glycol, a mixture of diisocyanates, and a chain extender and then melt-spun, dry-spun or wet-spun to form the spandex fiber. Another type of elastomeric yarn is polybutylene-terephtalate (PBT) yarn, a highly elastic, friction-texturized polyester yarn available from Swicofil AG Textile Services of Emmenbruecke, Switzerland.
Knitting spandex yarns together with non-elastomeric yarns in a jersey knitting process is sometimes referred to as “plating,” in which the non-elastomeric yarn and the bare spandex yarn are kept in a parallel, side-by-side relation throughout the knit, with the relation between the two yarns controlled such that the spandex material is always kept on one side of the non-elastomeric yarn. In plush or pile materials, the spandex yarn is generally kept away from the technical face of the fabric (i.e., the side opposite the raised pile), so as to present the typically more attractive and comfortable non-elastomeric yarn material at the technical face and to protect the spandex yarn fibers from snagging. Spandex yarns may also accept dye differently than other yarn materials, resulting in unacceptable color variations if exposed on the fabric surface. Stretchable pile fabrics may be made in a three end knitting process, meaning that three separate yarns are brought into the machine and knit together to form the fabric: a non-elastomeric ground yarn, an elastomeric ground yarn, and a pile yarn. As knit, the non-elastomeric and elastomeric ground yarns are generally limited to the ground of the fabric, and the pile yarns extend out of the fabric to form discrete loops, which in some cases are cut or shaved after processing to form a bed of fiber ends. The non-elastomeric and elastomeric ground yarns may be plated to keep the elastomeric yarns away from the technical fabric face while being knit into the fabric, or may be “laid in” or tucked into the knit structure using needle selection cams, to trap the elastomeric yarns between the non-elastomeric ground and pile yarns.
Some knit materials are formed as circular knit materials, meaning that they are initially knit as a tube on a machine in which the knitting needles are organized into a circular knitting bed. The needles are sequentially activated about the circular bed, such as by a cam surface acting against butt ends of the rotating set of needles, to lift and accept a yarn fed from a spool into a yarn carrier plate, to form a spiral row of stitches about the end of the tube. Such a process is also referred to as circular weft knitting. To circular knit a three end stretchable plush or pile fabric, the non-elastomeric ground yarn, the elastomeric ground yarn and the pile yarn are each fed separately to respective holes or slots in the carrier plate. In particular, the elastomeric yarn is kept separate from the non-elastomeric ground yarn until the point of introduction to the needles, so as to maintain the strict positional relation of non-elastomeric and elastomeric yarns, in order to keep the spandex material from being exposed, or “grinning through” the technical face of the fabric. In some cases, such as on some Orizio machines made by Orizio SRL, Brescia, Italy, the elastomeric yarn is run outside of the carrier plate, and guided into the needle before it closes by an outside guide roll. Circular knitting machines are also available from Vanguard Supreme, a division of the Monarch Knitting Machinery Corporation, in Monroe, N.C.
Improvements in stretchable laminate constructions and methods of making them will hopefully result in further advances in comfort and usefulness, as well as in reductions in costs.